sync() causes all pending modifications to filesystem metadata and
cached file data to be written to the underlying filesystems.
syncfs() is like sync(), but synchronizes just the filesystem
containing file referred to by the open file descriptor fd.

Since glibc 2.2.2, the Linux prototype for sync() is as listed above,
following the various standards. In glibc 2.2.1 and earlier, it was
"int sync(void)", and sync() always returned 0.
According to the standard specification (e.g., POSIX.1-2001), sync()
schedules the writes, but may return before the actual writing is
done. However Linux waits for I/O completions, and thus sync() or
syncfs() provide the same guarantees as fsync called on every file in
the system or filesystem respectively.

This page is part of release 4.13 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 SYNC(2)